Signal transmission with secrecy



June 8, 1965 M. E. MOHR SIGNAL TRANSMISSION WITH SECRECY 2 Sheets-Sheet2 Filed DeC. 20, 1943 @son FIG. 2

/Nl/ENTo/P M. E. MOH/P HVJ ATTORNEY 3,188,390 SGNAL TRANSMSS'N Milton E.Mohr, Summit, NJ., assigner to Beit 'eiephone Laboratories,incorporated, New York, RLY., a corporation et New York Fiied Dec. 20,1%3, Ser. No. 514596 5 Ciaims. {CL 179-1.5)

The present invention relates to message transmission with privacy andis in the nature of a modiiication of and improvement upon the privacysystem disclosed in the application of Lundstrom and Schimpf, Serial No.456,322, iiled August 27, 1942.

in that application disclosure, input speech waves are first analyzed,as in the vocoder, into pitch-defining and amplitude-defining orspectrum currents existing in separate circuits; secret key waves areadded to these various currents, and the resulting waves are put throughreentry circuits and, in one form or" circuit, through output Steppers.The output Steppers comprise groups of gasilled tubes whose grids areexpose for periodic briei instants of time to the Waves from the reentrycircuits to cause varying numbers of the tubes in a group to iire atdifferent times depending upon the instantaneous amplitude of the wavesapplied to the stepper grids at the instants of exposure. In this way,stepped output waves are produced. In order to restore the gasdilledstepper tubes vto normal just prior to each new exposure, their platevoltage is driven to zero for a short interval. This results in shortspaces of zero current between the impulses of output current. In theLundstrom-Schimpf disclosure, for example, the current puises haveuniform length of 14 milliseconds and are separated by 6 millisecondspaces.

in some cases such as in radio transmission in which certain fadingeffects are encountered, it may be desirable to produce for transmissiona stepped wave which does not fall periodically to zero but which is anuninterrupted current of stepped form.

This may be especially true where the stepped waves or impulses are tobe used forl frequency modulating carrier or subcarrier waves, in whichcase the spaced pulses produce greater excursions of frequency than doesan uninterrupted, stepped wave.

The object of this invention is to convert pulses with intervening dipsor spaces into a stepped wave in which the dips or spaces areeliminated.

in the form of the invention to be disclosed herein, this object isaccomplished by sampling the pulses at their peak amplitudes and placinga proportionate charge on a condenser, which is maintained until thenext sampling time when the charge is made proportional to the pulseamplitude existing at that sampling instant. The voltage across thiscondenser varies with time in the form of the stepped wave which isdesired.

The invention will be more clearly understood from the followingdetailed description taken in connection with the accompanying drawingsin which:

FGS. l and 2, when placed beside each other with FIG. l at the left,show a schematic circuit diagram of a transmitting terminal of the typeshown in the Lundstrom-Schimpf application modied to incorporate theimprovement feature constituting the present invention; and

FIG. 3 shows diagrams of current versus time to be referred to in thedescription.

In the description which follows, all ot that part ot the system whichis disclosed in detail in the Lundstrom and Schimpf application will beonly brietly sketched in order to afford a setting for describing indetail the present invention. In the drawing, the same reference nuidd@Fatented .inne t3, i965 metals are used to designate elements as areused in the Lundstrom-Schimpf disclosure for those circuit elements thatare common to both disclosures.

Speech waves from microphone 11 or other input circuit are impressed onthe analyzer shown as consisting of a pitch channel and a member ofspectrum channels, such as ten, of which only two are indicated. Thepitch channel includes iilter 14, rectifier 15, frequency measuringcircuit 16 and low-pass filter 17, and the spectrum channels includeiilter 18, rectifier 19 and low-pass iilter 21, all as shown bysimilarly numbered elements in the Lundstrom-Schimpf disclosure. Each ofthese channels, following the analyzer, includes an amplifier Z2 whichin practice may be a magnetic amplifier as in the Lundstrorn- Schimpfdisclosure. it will be understood that up to the amplifiers 22 thesechannels each carry a direct current of slowly varying amplitude, themaximum frequency component present being about 25 cycles. The amplifiedchannei currents are impressed on the message Steppers Zu@ which convertthe channel currents to square pulses of varying amplitude. Thesecurrents have added to them key pulses also of square pulse iorm comingfrom the output terminals of a key stepper Ztii associated with eachmessage stepper. The key Steppers are fed with key material from record1t?, the key for each channel being selected on a frequency basis bymeans of individual filters In the reentry circuit the summations otmessage plus key pulses are left unchanged so long as they are not inexcess of a given amplitude corresponding to about the maximumlamplitude or" the message pulses themselves. if they exceed thisamplitude they are reduce-d by a fixed amount such that the resultantpulses in the output of the reentry circuit occupy no greater amplituderange than the message pulses do at the output oi the message Steppers.The currents after reentry are amplified at 287 and sent into the outputsteppers which reform the impulses into better form for transmission.The pulses appearing at the output sides of these output Steppers, asalready noted, are of 14 milliseconds duration separated by6-millisecond spaces, and the pulses vary in amplitude in iixed steps.The form ot these pulses is indicated in FIG. 3, at a.

The timing of the Steppers 26d, 261 and the output Steppers iscontrolled from a 50-cycle wave derived from the record 1t?. Two wavesof a few hundreds of cycles frequency separated by 50 cycles frequencydifference are selected by iilter 25d and beat together in detector 251to derive a 50'cycle wave which is selected by iilter 2352 and impressedon exciter circuit 206. The latter controls the cathode impulser 2M andgrid impulser 205 to supply interrupted voltages to the plate and gridcircuits of the stepper tubes to cause them to be exposed for iiring, atcertain times, in response to the input voltage waves and to terminatethe pulse period by interrupting the plate current. The output Steppersare similarly controlled from exciter 23d, grid impulser 231 and cathodeimpulser 232. Since there is a time displacement between the exposureand restoring instants for the two tandem sets of Steppers, a phaseshifter 223is used to permit of such displacement while still using thesame SO-cycle timing wave.

in the Lindstrom-Schimpf disclosure the output pulses from the outputSteppers are applied directly to the frequency modulation oscillators263. in accordancevwith the present invention, certain apparatus isinserted between these two points of the system to provide foreliminating the dip or space between pulses. This apparatus will now bedescribed.

This apparatus is shown for convenience as contained within boundarylines or boxes 253, 251i and 255 for each aisance u) channel, togetherwith an exciter circuit 270 and pulsing supply circuit 280' common toall channels.

Before describing this apparatus in detail reference will first be madeto FIG. 3. As stated, the output step per output current is indicated ata as consisting of 14- millisecond pulses of varying height spaced apart6 milliseconds. Since the spaces are to be eliminated, the pulses whenreformed in accordance with this invention will be 20 milliseconds inlength and will step directly from one height to the next `as indicatedat c (except for a slight rounding of corners due to reactances in thecircuit). This is done by, in effect, exposing a condenser to the pulsethroughout its middle portion, say for l milliseconds, and then cuttingol the exposure for the next l() milliseconds allowing the condenser tohold its charge for this second lO-millisecond period. The condenser isthen exposed to the next pulse for milliseconds, and so on. Theseexposure and cut-off times are indicated by the positive halves of thewave shown at b. It is seen that the condenser storage bridges over the-millisecond spaces between the a pulses. The voltage appearing acrossthe condenser is the desired stepped, uninterrupted wave and this isindicated at c.

Reverting to FIG. 2, the condenser in question is condenser 260 in box253 and it is exposed to the voltage existing across resistor 218 in theoutput side of the output stepper whenever the pairs of diodes 262 arethrown to low impedance condition by voltage applied to them fromtransformer 263. At all other times they have high impedance. Thevoltage applied to the diodes is of square wave form, shown at b in FG.3, and is derived via apparatus in box 254 from pulsing circuit 28) overlead 284, as will be more fully described. As shown by comparison ofcurves a and b the diodes become conducting just after the grid exposurepulse on the output steppers has established a new signal value. 10milliseconds later they become non-conducting due to the reversal of thesquare wave voltage, and remain so until the next cathode impulser andgrid impulser pulses have occurred and a t new signal value has beenestablished. During the nonconducting interval the condenser holds itscharge unchanged, and at the beginning of the next conducting intervalit quickly assumes a new Vvoltage corresponding to the new signal value.

The circuit shown in box 254 is for the purpose of permitting the same-l-lSO-voltV pulsing supply 280 to serve a plurality such as eleven ofsuch circuits in parallel, and deliver a satisfactory square wave shapeto each. To obtain isolation of the eleven circuits, it is necessary tofeed the pulsed wave to them through transformers (263). The pulsingregulated power supply 280 (to be described presently) has a very lowimpedance during the conducting interval and a high impedance during thecut-off interval, which, when operating into a reactive load, such astransformer 263 and condenser 269, would distort the wave shape.Therefore, the controlled plate resistance of the tube 265 is used toprovide a relative low impedance during this cut-off interval withoutrequiring that this resistance remain across the pulsing supply duringthe conducting interval. This is accomplished by .using another winding268 on the transformer 263 which drives the grid of the tube 265negative when the pulsing supply is conducting and positive when thesupply is cut off. Since, however, there is a small delay between thetime the pulsing supply becomes conducting and the time the tube cutsoff, it is necessary to include a small delay in the tube plate circuitto avoid momentarily overloading the pulsing supply. The inductance 266is included for this purpose.

The resistance 267 is included as a means of balancing the directcurrent ampere turns in the secondary winding transformer at 263 withoutdanger of saturation and wave lowingV an equivalent and opposing currentto iiow in the primary winding. This permits the use of a small sizedtransformer at 263 without danger of saturation and wave shapedistortion. Resistances 269 limit the diode rectification currents.

In one case where the pulsing supply was interrupted at cycles persecond, the condenser 269 and a value of 4 microfarads, resistor 267 wasof the order of 60,000 ohms and the transformer 263 was of the smallinput type wound to have high impedance. These values are not to betaken as limiting but are given by way of example and can be variedwidely to suit conditions.

The exciting circuit 270 comprises a pair of pentodes 271 and 272supplied with plate and screen voltage from potentiometer resistance 88connected across the filtered output of rectitier 87 fed from powersource 63. The wave received through phase shifter 223 (FIG. l) is, asstated, a 50-cycle wave each half-cycle of which is, therefore, ofIO-milliseconds duration. Tube 271 is conducting at all times exceptwhen cut off by tube 272, which oc-` curs every other half-cycle orevery other l0 milliseconds period. The phase at which tube 272 startsto conduct is fixed with respect to the output stepper operate times bymeans of phase shifter 273 comprising series resistance and shuntcapacity in proper proportion to give the desired relative timing. Asthe control grid of tube 272 is driven in the positive direction thetube begins to pass current which flows (negatively) from the platethrough lead 274 and resistor 275 to the 40G-volt point on resistor 88,to the SOO-volt point and to the cathode of tube 272. This current owingthrough resistor 275 cuts otf tube 271. At the end of a half-cycle (10milliseconds) the wave reverses on the control grid of tube 272 reduc-.ing the current through the tube to zero and tube 271 becomesconducting, its space current tiowing from the plate through lead 264and through resistors 281 and 282 in series to ground 100 at one end ofresistor 88 and to the cathode of tube 271 at -400 volts. This causes ahighly negative voltage to be applied to lead 264 for l0- millisecondperiods, this volta-ge being sutiicient to swing the grid of tube 286beyond cut-oit and interrupt the current supplied to conductor 284 fromrectiiier 283.

The pulsing circuit 280 comprises a power supply in the form of analternating current source 63 and rectifier 283 followed by a iilter 285and regulating and switching tube 286. Ordinarily the tube 286 ispassing current to the terminal resistance 287 one terminal of which isconnected to lead 284 and the other terminal of which is Y connected toground. Tube 288 acts as a measuring device to detect smal-lfluctuations in voltage in resistance 287 and control the impedance ofregulating tube 286 in such direction and to such extent as to hold theterminal voltage across resistor 287 closely constant. Battery 289 isconnected between the control grid of tube 288 and a tap point onresistor 287 of nominally the same (but opposite) voltage to ground asthe voltage of the battery. The plate of tube 288 derives positivevoltage from the positive terminal of rectifier 283 through seriesresistor 281. When the terminal voltage across resistor 287 has normalvalue the tube 288 is placing such a bias on the grid of regulator tube286 as to maintain the normal sup-l ply of current to resistor 287, inthe absence of a blocking from tube 271 voltage on lead 264.Applications of negative voltage to lead 264 by tube 271 as describedcause tube 286 to interrupt the current and deliver pulses to lead 284.This pulsing circuit in and of itself forms no part of the presentinvention but is claimed in the applicat1'on of another.

It will be noted that when diodes 262 are in their conducting condition,they afford a bilaterally conducting low impedance path between resistor28 and condenser 269. If the voltage existing across resistor 218'ishigher than the terminal voltage across the condenser 268,V chargingcurrent flows into the condenser to yraise its voltage to equal thatacross resistor 218. If the condenser voltage exceeds the voltage acrossresistor 218, some condenser discharge current flows back through thediodes (and is dissipated in the circuit resistances) to equalize thevoltage across the condenser to that across resistor 218. The resistance261 is inserted to give the condenser circuit a time constant of about lmillisecond since this is found to improve transmission through fadingconditions where radio transmission is used.

The voltage across condenser 266 is applied to the grid of a directcurrent amplifier 290 the plate circuit of which includes the modulatinginductance 222 of the frequency modulated oscillator 263 of the typedisclosed more fully in the Lundstrom-Schimpf application. The use ofthis amplifier insures that the load impedance connected acrosscondenser 26) is so high as not to discharge the condenser but to allowit to maintain its charge during the itl-millisecondi storage period.This amplifier 29u has large stabilizing resistances including 291 andthe other resistances shown between the cathode and ground. The loadcircuit through winding 222 is connected across resistance 291 andbattery 292. The proportioning of the elements is such that the currentin the winding 222 varies between zero and some negative quantitycorresponding to the limits of the current variations in resistor 2l8.

Instead of using dilerent normal frequencies in the frequency modulatedoscillators 263 in the different channels, all oscillators use the samenormal frequency and the frequency modulated output bands are sent intoamplitude modulators 284 which step the channel band frequencies tosuccessively higher levels to space them properly in the frequencyspectrum for suitable transmission. For this purpose a source of basefrequency waves 399 of highly constant frequency and a harmonicgenerator 361 are used to supply channel shifting frequencies to thevarious shifting modulators 294 through selecting filters 302. This useof the same frequency for all of the frequency modulated oscillatorstogether with the channel shifting modulators forms no part of thepresent invention.

The filters 153 select one side-band of the modulated output waves frommodulators 294 for transmission over the line or channel 154 which maybe the actual transmission path to the distant receiver station or maylead to a radio transmitter or other type of transmission channel.

The receiving terminal is not illustrated since it may be the same asthat disclosed in the Lundstrom-Schimpf application. There may be needof a different proportioning of certain of the low-pass filters ordifferent adjustments but the circuit arrangement can be the same.

What is claimed is:

1. In a privacy communication system in which coded message waves are inthe form of impulses of diiering amplitude with equal spaces betweenthem, means for transforming such waves into an uninterrupted outputvoltage wave of stepped amplitude comprising means to sample the codedmessage Waves at equal intervals at their peak amplitudes, means toplace a charge on a condenser that is proportional to the coded messagewaves at the instants of sampling, and means for continuously taking olfas output the voltage existing across said condenser.

2. In a transmission system comprising parallel channels carryingvarying amplitude currents, means in each channel to convert saidcurrents to uninterrupted stepped currents comprising in each channel acondenser and a valve circuit for operatively connecting said condenseracross the channel at periodic intervals to be charged under control ofsaid first-mentioned currents, a common timing circuit for the valvecircuits of all of said channels, for supplying a timing voltage fortiming said intervals and means in each channel for preventingdistortion of the timing voltage by interaction from the other channels.

3. In a transmission system including a plurality of channels eachcarrying impulses of varying strength separated by spaces of no current,a circuit for converting the impulses in each channel to continuouscurrent comprising in each channel a bilaterally conducting variableresistance device connected in series in the channel followed by acondenser shunted across the channel, and means common to said channelsfor making the resistance of each of said devices low during theexistence of said pulses to permit a proportionate voltage to bedeveloped across the respective condenser and for making the resistanceof each of said devices high during the existence of said spaces toretain the charges on said condensers.

4. In a transmission system including a plurality of channels eachcarrying impulses of varying strength separated by spaces of no current,a circuit for converting the impulses in each channel to continuouscurrent cornprising in each channel ya voltage responsive variableresistance device in series relation in the channel followed by acapacity connected across the channel and a common control circuit forchanging the resistance of said devices simultaneously between asubstantially non-conducting value and a negligibly low value, means tosupply voltage pulses over said common circuit to all of said devices tochange their resistance comprising a twowinding transformer per channel,connected between said common circuit and an individual device, forconductively isolating said devices from said common circuit, and meansto control the shaping of the voltage pulses transmitted through saidtransformers comprising a space discharge device having its space pathconnected across said common circuit `and meansto control its impedancein accordance with a voltage developed in a Winding of saidtransformers. Y

5. In a privacy system including a signal analyzer circuit comprising aplurality of channels each including a secret key combining circuitfollowed by an output stepper for delivering output pulses of varyingstrength with intervening spaces, a circuit for changing the outputcurrent to continuous current of stepped wave form comprising a devicecapable of varying its resistance from practically infinite topractically zero Value in response to applied voltages, connected inseries relation in each vchannel followed by a condenser connectedacross said channel, an outgoing circuit connected across saidcondenser, and means to control said devices comprising a common sourceof pulsing voltage supply, transformers individual to said channels andcoupling said common source to said devices to transmit control pulsesto the latter, a pulse-shaping circuit for each transformer comprising aspace discharge tube having its space path connected across said commonsupply source and means to vary its impedance in accordance withsecondary voltage variations on the corresponding transformer.

No references cited.

ROBERT H. ROSE, Primary Examiner.

1. IN A PRIVACY COMMUNICATION SYSTEM IN WHICH CODED MESSAGE WAVES ARE INTHE FORM OF IMPULSES OF DIFFERING AMPLITUDE WITH EQUAL SPACES BETWEENTHEM, MEANS FOR TRANSFORMING SUCH WAVES INTO AN UNINTERRUPTED OUTPUTVOLTAGE WAVE OF STEPPED AMPLITUDE COMPRISING MEANS TO SAMPLE THE CODEDMESSAGE WAVES AT EQUAL INTERVALS AT THEIR PEAK AMPLITUDES, MEANS TOPLACE A CHARGE ON A CONDENSER THAT IS PROPORTIONAL TO THE CODED MESSAGEWAVES AT THE INSTANTS OF SAMPLING, AND MEANS FOR CONTINUOUSLY TAKING OFFAS OUTPUT THE VOLTAGE EXISTING ACROSS SAID CONDENSER.